In recent years, in the production of semiconductor elements and liquid crystal display elements, advances in lithography techniques have lead to rapid progress in the field of miniaturization.
Typically, these miniaturization techniques involve shortening the wavelength of the exposure light source.
Conventionally, ultraviolet radiation typified by g-line and i-line radiation has been used, but nowadays KrF excimer lasers (248 nm) are the main light source used in mass production, and ArF excimer lasers (193 nm) are now also starting to be introduced in mass production.
Furthermore, research is also being conducted into lithography techniques that use F2 excimer lasers (157 nm), EUV (extreme ultraviolet radiation), and EB (electron beams) and the like as the light source (radiation source).
Resists for use with these types of short wavelength light sources require a high resolution capable of reproducing patterns of minute dimensions, and a high level of sensitivity to these types of short wavelength light sources.
One example of a known resist that satisfies these conditions is a chemically amplified resist, which includes a base resin and an acid generator (hereafter referred to as a PAG) that generates acid on exposure. These chemically amplified resists include positive resists in which the alkali solubility of the exposed portions increases, and negative resists in which the alkali solubility of the exposed portions decreases.
Until recently, polyhydroxystyrene (PHS) or derivative resins thereof in which the hydroxyl groups are protected with acid-dissociable, dissolution-inhibiting groups (PHS-based resins), which exhibit high transparency relative to a KrF excimer laser (248 nm), have been used as the base resin component of chemically amplified resists.
However, because PHS-based resins contain aromatic rings such as benzene rings, their transparency is inadequate for light with wavelengths shorter than 248 nm, such as light of 193 nm. Accordingly, chemically amplified resists that use a PHS-based resin as the base resin component suffer from low levels of resolution in processes that use light of 193 nm n.
As a result, resins that contain structural units derived from (meth)acrylate esters within the principal chain (acrylic resins) are now widely used as base resins for resists that use ArF excimer laser lithography, as they exhibit excellent transparency in the vicinity of 193 nm.
Further, as positive resists, as shown in Patent Document 1, resins having a structural unit derived from a tertiary ester compound of (meth)acrylic acid such as 2-alkyl-2-adamantyl (meth)acrylate as a structural unit having an acid dissociable, dissolution inhibiting group are mainly used.
In these resins, it is known that the dissolution energy of the acid dissociable, dissolution inhibiting group is high, and hence, the types of usable acid generators are limited. Specifically, these resins had problems in that, when an acid generator which is capable of generating a strong acid such as an onium salt having a fluorinated alkylsulfonate ion as the anion moiety is not used, the acid dissociable, dissolution inhibiting groups cannot be satisfactorily dissociated. As a result, the resin could not function as a resist.
Further, in recent years, as resins having an acid dissociable, dissolution inhibiting group which are usable in ArF excimer laser lithography and the like, attention has been drawn to resins having a structural unit in which the hydrogen atom of (meth)acrylic acid is replaced by an acetal group such as 1-alkoxyalkyl group (see Non-Patent Document 1).
[Patent Document I] Japanese Patent (Granted) Publication No. 2,881,969
[Non-Patent Document l] J. Photopolym. Sci. Technol. 17 (2004) 483-488